North Wales. 427 



near Geneva, boulders shed from the huge glacier may 

 still be seen with their tops above water, and in the 

 midst of the Delta of the Rhone, between Bex and the 

 mouth of the river, portions of an old terminal moraine 

 peep through the wide alluvial flats and marshes which 

 began to be deposited by its waters, at a time when the 

 lake was several miles longer than at present. 



The gradual retreat of the glacier of the Pass of 

 Llanberis is further proved by numerous perched blocks, 

 which, here and there, isolated or in groups, stand on 

 the surfaces of roches moutonnees, as, for example, at 

 Pont-y-gromlech, and in many other places, masses of 

 stone that, so to speak, floated on the surface of the ice, 

 were left perched upon the rounded rocks in a manner 

 somewhat puzzling to those who are not geologists ; 

 for they lie in places to which they clearly cannot 

 have rolled from the mountains above, because their 

 resting places are separated from it by a hollow ; 

 and, besides, many of them stand in positions so pre- 

 carious, that if they had rolled from the mountains, 

 they must, on reaching the points where they lie, 

 have taken a final bound and fallen into the valley 

 below. But when experienced in the geology of 

 glaciers, the eye detects the true cause of these pheno- 

 mena, we have no hesitation in coming to the conclu- 

 sion that, as the glaciers declined in size, the errant 

 stones were let down upon the surface of the rocks so 

 quietly and so softly, that there they will lie until an 

 earthquake shakes them down, or until the wasting of 

 the rock on which they rest precipitates them to a 

 lower level. Finally, the climate still ameliorating, 

 the glaciers shrunk farther and farther into the heart 

 of the mountains, until, at length, here and there, in 

 their very uppermost recesses, we find the remains of 



